Numerous apparatuses have been proposed for preventing over-filling of fuel tanks. The most common used method is an automatic cut-off within a nozzle. Typically this automatic cut-off uses a vapor path from the nozzle outlet back to a venturi around the fuel flow path within the nozzle. A sufficiently high pressure must be maintained at a point within this path to indicate that vapor is being drawn into the vapor path rather than liquids. When liquids enter the vapor path, the pressure drop in the path increases, and the pressure at the sensor point will decrease. When this pressure decreased below a threshold pressure, the fuel flow is cut-off, usually by a mechanical trip. As a back-up to this fuel cut-off switch, fuel pumps with a vapor recovery system usually also include a fuel cut-off triggered by high level of amperage being drawn by a vacuum pump drawing vapors from the fuel tank. This system is undesirable because fuel will fill a considerable volume of the vapor recovery system prior to the vacuum pump drawing a high level of amperage. The shut-down is therefore considerably slower than what would be desired, resulting volumes of liquid hydrocarbons to be contended with to prevent the hydrocarbons from being unwanted emissions.
It is therefore an object of the present invention to provide a method for determining the volume of a fuel tank that is not occupied by liquid. In another aspect it is an object to provide such a method that can be used in conjunction with a refuelling process to minimize or eliminate any emissions of fuel caused by splash-back when the tank fills.